MDNR Warns Of Spreading Oak Wilt Through Pruning

From Michigan’s Department of Natural Resources:

Avoid oak wilt: Don’t prune or injure oak trees during greatest risk period.

Have an oak tree on your property? To keep it healthy, don’t prune it from mid-April through the summer. That’s a key time for infection with oak wilt, a serious disease that can weaken white oaks and kill red oak trees within weeks.

Oak wilt, caused by a fungus, has been reported throughout the Midwest, including Michigan, said Ryan Wheeler,  invasive species biologist with the Michigan Department of Natural Resources.

Red oaks are most susceptible to the disease. These trees have leaves with pointed tips and include black oak, northern red oak and northern pin oak. Trees in the white oak group have rounded leaf edges and include white oak and swamp white oak. They are less susceptible.

Symptoms most often appear from June until September.

“Affected trees will suddenly begin to wilt from the top down, rapidly dropping leaves, which can be green, brown or a combination of both colors,” Wheeler said.

Oak wilt is spread above ground mainly by sap-feeding beetles that carry the disease spores from an infected tree, or wood cut from an infected tree, to fresh wounds, including pruning cuts, on healthy trees. The infection also spreads below ground, through root grafts among neighboring trees.

The highest risk of infection occurs April 15-July 15, but it is prudent to avoid pruning or injuring oak trees until they have lost leaves for the winter, typically from November through mid-March, Wheeler said. If you must prune or remove oaks during the risk period, or have a tree that gets damaged, immediately cover wounds with tree-wound paint or latex-based paint.

Don’t move firewood, especially if it comes from oak wilt-killed trees, as it can harbor the fungus. If you suspect your firewood is tainted by oak wilt, cover it with a plastic tarp all the way to the ground, leaving no openings. This keeps beetles away so they can’t move spores from the firewood to otherwise healthy trees. Once the firewood has been cut long enough, to the point where all of the bark loosens, the disease can no longer be spread.

If you suspect your oak trees have this disease:

  • Get help from an oak-wilt qualified specialist. Visit www.MichiganOakWilt.org for a listing and more information.
  • Michigan State University’s Diagnostic Clinic can verify infection. Find instructions at https://pestid.msu.edu/  or call 517-355-4536.

Report infections to DNR-FRD-Forest-Health@michigan.gov or by phone at 517-284-5895; you also can use the MISIN website or mobile app.

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